Exposure method

ABSTRACT

An exposure method for exposing a workpiece in a proximity exposure system, includes a first exposure step for printing, by exposure, an image of a first mask pattern on a predetermined portion of the workpiece, and a second exposure step for printing, by exposure, an image of a second mask pattern, different from the first mask pattern, on the predetermined portion of the workpiece, wherein exposures in the first and second exposure steps are performed superposedly, prior to a development process.

FIELD OF THE INVENTION AND RELATED ART

[0001] This invention relates to an exposure method and, moreparticularly, to a proximity exposure method using X-rays, for example.The exposure method of the present invention is suitably applicable tomanufacture of various microdevices such as a semiconductor chip (e.g.,IC or LSI), a display device (e.g., liquid crystal panel), a detectingdevice (e.g., magnetic head), and an image pickup device (e.g., CCD),for example.

[0002]FIG. 1 shows an example of an X-ray proximity exposure apparatusof known type (Japanese Laid-Open Patent Application No. 2-100311).Denoted in the drawing at 1 is an X-ray source (light emission point)such as a synchrotron orbital radiation (SOR), and denoted at 2 is a SORX-ray beam being expanded in an X direction into a slit-like shape.Denoted at 3 is a convex mirror, made of SiC, for example, for expandingthe slit-like X-ray beam 2 in a Y direction. Denoted at 2 a is the X-raybeam having been expanded by the convex mirror 3 into an area shape.Denoted at 7 is a workpiece to be exposed, such as a semiconductor waferhaving been coated with a resist, for example. Denoted at 10 is a mask.Denoted at 4 is a beryllium film for isolating an ambience at the SORside and an ambience at the mask (and workpiece) side from each other.Denoted at 5 is a focal plane type shutter being provided for exposureamount adjustment. In an exposure operation, the mask 10 and theworkpiece 7 are placed with a spacing (gap) of about 10 micronsmaintained therebetween. As the shutter 5 is opened, a slit-likehigh-luminance X-ray beam 2 from the SOR, for example, and beingexpanded into an area shape (X-ray beam 2 a) by the convex mirror 3, isprojected to the mask 10 and then to the workpiece 7, by which a patternimage of the mask 10 is transferred to the workpiece 7 at a unitmagnification.

[0003] As regards the X-rays in this case, a wavelength of about 0.5-20nm is used. Therefore, in connection with the wavelength only,theoretically, a very high resolution of 0.05 micron (50 nm) or lesswill be obtainable. Practically, however, such a high-resolution maskitself is difficult to manufacture. If a mask of a nominal smallestlinewidth of 0.05 micron is manufactured by use of a technique forproduction of a conventional mask of smallest linewidth of 0.1 micron(100 nm), any positional error or any error in the line-and-space(linewidth and spacing) of a pattern produced will be transferred to aworkpiece as a mask defect. It will cause a void in the pattern to beformed, or a positional deviation of the pattern. Further, a producedmask pattern may not have a proper linewidth or a sufficient thickness.In these occasions, a sufficient contrast will not be attainable, andthe pattern will not be resolved satisfactorily.

SUMMARY OF THE INVENTION

[0004] It is accordingly an object of the present invention to providean exposure method by which a pattern can be formed at a higherresolution and a higher precision, on the basis of a currently availableX-ray exposure apparatus and a mask which can be produced in accordancewith a current technique.

[0005] It is another object of the present invention to provide anexposure method which enables accomplishment of resolution even in astrict condition under which the contrast is too low and the resolutionis currently difficult to accomplish.

[0006] These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a schematic view of an X-ray proximity exposureapparatus of a known type.

[0008]FIG. 2A is a graph of an exposure intensity distribution for afine pattern, for explaining the principle of a dual exposure processaccording to an embodiment of the present invention.

[0009]FIG. 2B is a graph of an exposure intensity distribution for arough pattern, for explaining the principle of the dual exposureprocess.

[0010]FIG. 3 is a graph of a combined exposure intensity distribution,for the fine pattern of FIG. 2A and the rough pattern of FIG. 2B.

[0011]FIG. 4 is a schematic view of a resist pattern which is obtainablethrough an exposure process with the exposure intensity distribution ofFIG. 3 and through a development process.

[0012]FIG. 5 is a graph of an exposure intensity distribution in a casewhere, in the graph of FIG. 3, the position of the rough pattern of FIG.2B is shifted.

[0013]FIG. 6 is a graph of an exposure intensity distribution in a casewhere, in the graph of FIG. 2B, the position of the rough pattern isshifted.

[0014]FIG. 7 is a schematic view of an example of a fine pattern maskand a fine pattern to be printed by use of that mask.

[0015]FIG. 8 is a simulation chart of an exposure intensity distributioncorresponding to the state of exposure of FIG. 7.

[0016]FIG. 9 is a graph of an example of an X-ray spectrum upon thesurface of a workpiece to be exposed.

[0017]FIG. 10 is a schematic view of another example of a fine patternmask and a fine pattern to be printed by use of that mask.

[0018]FIG. 11 is a simulation chart of an exposure intensitydistribution corresponding to the state of exposure of FIG. 10.

[0019]FIG. 12 is a simulation chart wherein an exposure intensitydistribution to be produced by a mask having the same pattern shape asthat of FIG. 10 and having an X-ray absorbing member of a differentmaterial and a different thickness, is simulated.

[0020]FIG. 13 is a schematic view for explaining production of a finepattern in a case where the fine pattern of FIG. 10 is exposed with anexposure gap different from that of FIG. 10.

[0021]FIG. 14 is a schematic view for explaining production of a finepattern in a case where the fine pattern of FIG. 13 and the same patternbut shifted by a half period are printed superposedly.

[0022]FIG. 15 is a simulation chart of an exposure intensitydistribution in the state of exposure of FIG. 13.

[0023]FIG. 16 is a simulation chart of an exposure intensitydistribution in the state of exposure of FIG. 14.

[0024]FIG. 17 is a simulation chart of an exposure intensitydistribution, for explaining the state of exposure to be provided by useof a rough pattern according to a first embodiment of the presentinvention.

[0025]FIG. 18 is a graph of an exposure intensity distribution whereinthe exposure intensity of FIG. 16 and the exposure intensity of FIG. 17are combined with each other, at a ratio of 0.5:1.

[0026]FIG. 19 is a simulation chart of an exposure intensitydistribution, for explaining the state of exposure to be provided by useof a rough pattern according to a second embodiment of the presentinvention.

[0027]FIG. 20 is a graph of an exposure intensity distribution whereinthe exposure intensity of FIG. 11 and the exposure intensity of FIG. 19are combined with each other, at a ratio of 0.5:1.

[0028]FIG. 21 is a simulation chart of an exposure intensitydistribution, for explaining the state of exposure to be provided by useof a rough pattern according to a third embodiment of the presentinvention.

[0029]FIG. 22 is, a graph of an exposure intensity distribution whereinthe exposure intensity of FIG. 16 and the exposure intensity of FIG. 21are combined with each other, at a ratio of 0.5:1.

[0030]FIG. 23 is a simulation chart of an exposure intensitydistribution, for explaining the state of exposure to be provided by useof an idealistic rough pattern according to a fourth embodiment of thepresent invention.

[0031]FIG. 24 is a graph of an exposure intensity distribution whereinthe exposure intensity of FIG. 8 and the exposure intensity of FIG. 23are combined with each other, at a ratio of 0.35:1.

[0032]FIG. 25 is a simulation chart of an exposure intensitydistribution, for explaining the state of exposure to be provided by useof a practical rough pattern according to the fourth embodiment of thepresent invention.

[0033]FIG. 26 is a graph of an exposure intensity distribution whereinthe exposure intensity of FIG. 8 and the exposure intensity of FIG. 25are combined with each other, at a ratio of 0.35:1.

[0034]FIG. 27 is a schematic view of a checkered pattern, as anotherexample of a fine pattern to be used in the present invention.

[0035]FIG. 28 is a schematic view of a grid-like pattern, as a furtherexample of a fine pattern to be used in the present invention.

[0036]FIG. 29 is a schematic view corresponding to a section taken alonga line A-A′ or B-B′ in FIG. 27, where an X-ray proximity exposure isperformed by use of a mask having the checkered pattern of FIG. 27.

[0037]FIG. 30 is a schematic and plan view for explaining an X-rayintensity distribution upon a resist 8 of FIG. 29, to be produced by theexposure process of FIG. 29.

[0038]FIG. 31 is a flow chart of microdevice manufacturing processes.

[0039]FIG. 32 is a flow chart for explaining details of a wafer processin the procedure of FIG. 31.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] Preferred embodiments of the present invention will now bedescribed with reference to the accompanying drawings.

[0041] In one preferred form of the present invention, a fine patternand a rough pattern are printed superposedly (superposing exposure).

[0042] A fine pattern may be a pattern like that of a diffractiongrating. A diffraction image of a mask pattern having a diffractiongrating shape which can be manufactured on the basis of a currenttechnique, may be printed by exposure. This enables that, by use of amask which can be produced by a current technique, a fine pattern havinga smallest linewidth, smaller than that attainable with an ordinaryX-ray exposure process, is printed at a high resolution.

[0043] As regards a rough pattern, a desired rough pattern may beprinted at a location where a fine pattern should be left.

[0044] A fine pattern and a rough pattern such as described above may beexposed respectively by individual exposure amounts, each being at alevel lower than the exposure amount threshold of a resist upon aworkpiece but, when combined, the sum of them being at a level higherthan the exposure threshold. As a result, a pattern can be formed onlyin such portion having been exposed superposedly. Namely, in thatportion, a pattern can be formed with a precision of the fine pattern.Here, either the fine-pattern exposure or the rough-pattern exposure maybe made first. A second mask for the rough pattern exposure may be onedifferent from a first mask for the fine pattern exposure, having anabsorptive member of a different material and/or a different thickness.They may be determined in accordance with conditions for accomplishing adesired rough pattern.

[0045] A desired rough pattern may be a pattern having a large openingor a small pattern produced in accordance with a conventional technique,for example. Alternatively, it may be a pattern based on a peak ofFresnel diffraction.

[0046] Namely, a rough pattern may be formed with an opening larger thana fine pattern, and a pattern corresponding to the position of theopening of the rough pattern may be transferred. By doing so, thecontrast and precision can be improved, still on the basis of aconventional mask manufacturing technique.

[0047] There is a further advantage that a tolerance for a variation inthe exposure amount or a tolerance for a variation in the exposure gap,for exposure of a small pattern (e.g., 0.05 micron line-and-space) asproduced on the basis of a conventional technique (e.g., 0.1 micronline-and-space), can be enlarged. Namely, the resolution can beimproved. In addition, even in a case where the contrast is too low tobe resolved only through the Fresnel diffraction of a rough pattern, theexposure process may be performed while a peak of Fresnel diffraction ofa rough pattern and a peak of Fresnel diffraction of a fine pattern arekept registered with each other, by which a high contrast can be assuredand the resolution can be improved.

[0048] The principle of a dual exposure process according to thisembodiment of the present invention will be described in greater detail,in conjunction with FIGS. 2A-4.

[0049] As an example, a fine pattern may be exposed or photoprinted withan exposure amount (1), as shown in FIG. 2A. Thereafter, as shown inFIG. 2B, a rough pattern having a linewidth corresponding to threeperiods of the fine pattern is photoprinted with an exposure amount (1).Then, as shown in FIG. 3, a portion where the fine pattern and the roughpattern are printed superposedly bears an exposure amount (2), whereas aportion exposed with either the fine pattern or the rough pattern onlybears an exposure amount (1). The remaining portion bears zero exposureamount (0). Therefore, as shown in FIG. 3, where the exposure thresholdDs is set between the exposure amounts (1) and (2), only the portionwhere the fine pattern and the rough pattern are printed superposedlycan be developed, as shown in FIG. 4. Here, the exposure amount (1) or(2) is referred to only for convenience in explanation, and it does nothave a specific physical significance.

[0050] Advantages of such dual exposure process will be described below.Broken lines in FIGS. 5 and 6 depict an exposure amount distribution inan occasion where the rough pattern of FIG. 2B is printed with adeviation from the correct transfer position (solid line position) inFIGS. 5 and 6 where the pattern should be printed correctly. If theposition of the rough pattern shifts such as described above, theexposure amount distribution changes slightly from the solid-linedistribution to be provided correctly. However, there occurs no changein the portion to be exposed with an exposure amount (2). Namely, evenif the rough pattern transfer position shift slightly, the resistpattern after a development process can be formed exactly at the sameposition as the case of FIG. 4 where the rough pattern transfer positionis not deviated.

[0051] In the dual exposure process of this embodiment, an exposureregion may be divided into zones in accordance with a fine pattern, anda range for divided zones may be selected in accordance with a roughpattern. The precision therefor is determined by the fine pattern, andthere is substantially no influence to the precision of a patternresulting, after development, from the rough pattern. Thus, there is alarge advantage that a pattern having been manufactured in accordancewith a conventional technique can be photoprinted at a high resolution,by use of a conventional X-ray exposure apparatus.

[0052] The inside area of the selected zones or a portion where only theprecision or resolution for the rough pattern is sufficient, may beexposed by use of a third mask (i.e., a second rough pattern mask) forX-ray transmission, so that the exposure threshold level may beexceeded. In that occasion, the (first) rough pattern mask and thesecond rough pattern mask to be used for the dual exposure may becombined into a single mask, that is, a rough pattern mask having alocally adjusted transmission factor, such that a desired pattern can beproduced through twice exposures only.

[0053] Next, a method of producing a high resolution fine pattern by useof a mask which can be produced on the basis of a conventionaltechnique, will be described.

[0054]FIG. 7 is a schematic view of an example of a fine pattern maskaccording to an embodiment of the present invention, and a fine pattern(X-ray exposure amount) to be printed by use of that mask and throughX-ray exposure. Denoted in the drawing at 10 is a fine pattern mask, anddenoted at 11 is a membrane. Denoted at 12 is an X-ray absorbingmaterial. The membrane 11 is made of a material having a high X-raytransmission factor, such as, for example, SiC or SiN of 2-micronthickness. The X-ray absorbing member 12 is made of tungsten, molybdenumor tantalum, for example. The X-ray absorbing member 12 on the membrane11 is formed in a diffraction grating pattern (e.g., a stripe-likepattern of a regular period), whereby the fine pattern mask 10 isprovided.

[0055] Here, the material and the thickness of the X-ray absorbingmember 12 as well as the gap (exposure gap) between the mask 10 and aworkpiece (e.g., a semiconductor wafer having a resist coating) in anexposure process, for example, may be set so that the exposure amountjust below the absorbing member 12 becomes zero. As X-rays are projectedunder such condition, a fine pattern (X-ray diffraction image) can beformed upon the workpiece. FIG. 7 illustrates a case where the patternformed on the mask 10 and the fine pattern thereof (a diffraction imageof the mask pattern) have the same period. Thus, if the mask pattern hasa period (pitch) of 0.1 micron, the fine pattern has also a period 0.1micron. As regards the period, in this case, if the line-and-space is1:1, the linewidth is 0.05 micron which is a half of 0.1 micronlinewidth as can be attained with the conventional mask manufacturingtechnique. Because of use of a diffraction image, however, errors inactual lines and spaces will be averaged, such that a fine patternhaving a highly uniform lines and spaces can be printed.

[0056]FIG. 8 is a graph chart of simulation of an exposure intensity(exposure amount) distribution to be absorbed by a workpiece 7 throughFresnel diffraction of X-rays, in an example where: the X-ray source 1(FIG. 1) comprises a synchrotron orbit radiation of 585 MeV and a radius0.593 m; the SiC mirror 3 has a glancing angle of 15 mrad; the berylliumfilm 4 has a thickness 18 microns; the mask 10 (FIG. 7) has a membrane11 of 2-micron thickness SiC on which an absorbing member 12 of02.5-micron thickness tungsten (W) is formed in a stripe-like pattern ofline-and-space of a 60 (nm):40 (nm) ratio; and the gap (exposure gap)between the mask 10 and the workpiece 7 is kept 6.2 microns. The X-rayspectrum impinging on the mask 10 is based on the value shown in FIG. 9.It is seen also from the result of simulation that a fine linear patternof 0.1 micron period can be produced with high contrast.

[0057]FIG. 10 is a schematic view of a mask pattern and a fine pattern,in a case for obtaining an exposure intensity distribution (finepattern) of a 1/2 period (pitch) of the line-and-space of a maskpattern, on the basis of X-ray interference. In FIG. 10, the exposuregap and the thickness of the absorbing member 12 are so set that theintensity below the absorbing member 12 and the intensity below theopening 13 are at the same level. In this example, while using a maskpattern of 0.2-micron period (0.1-micron line-and-space) which can bemet by a conventional technique, a fine linear pattern of 0.1-micronperiod (0.05 line-and-space) can be printed. Namely, by using a mask ofa smallest linewidth that can be met by a conventional technique, a finelinear pattern of a half linewidth can be printed.

[0058] Moreover, a diffraction image of a 1/n period (n is an integernot less than 3) of the line-and-space of the mask, may be produced.

[0059]FIG. 11 is a graph chart of simulation of an exposure intensity(exposure amount) distribution of an X-ray diffraction image to beabsorbed by a workpiece 7, under the same conditions as of FIG. 8 exceptthat: the absorbing member 12 of the mask is made of 0.40-micronthickness tungsten; the line-and-space of the mask pattern is at a ratioof 0.1 (micron):0.1 (micron); and the exposure gap is 32 microns. FIG.12 is a graph chart of simulation of a distribution (exposure amountdistribution) of an X-ray diffraction image to be absorbed by aworkpiece 7, under the same conditions as of FIG. 11 except that: theabsorbing member 12 of the mask is made of 0.70-micron thicknesstantalum (Ta); and the exposure gap is 20 microns. It is seen from theresults of these simulations that, where the material and the thicknessof the absorbing member 12 as well as the exposure gap are setappropriately, a fine linear pattern of 0.1-micron period can beproduced with high contrast.

[0060]FIG. 13 is a schematic view of another example of a mask patternand a fine pattern, in a case for obtaining an exposure intensitydistribution (fine pattern) of a 1/2 period of the line-and-space of amask pattern, on the basis of X-ray interference. In FIG. 13, theexposure gap and the thickness of the absorbing member 12 are so setthat the X-ray intensity upon the workpiece, at every half period of theline-and-space of the mask pattern, becomes substantially equal to zero.In a case where the line-and-space of the mask pattern is at a unitratio (same magnitude), they may be so selected that the intensitiesbelow the absorbing member 12 and below the opening 13 becomesubstantially equal to zero. As regards the exposure process, the firstexposure may be performed in the state shown in FIG. 13, and the secondexposure may be made in the state shown in FIG. 14 wherein the mask isshifted by a half period. As a result, as shown in FIG. 14, the totalexposure amount (solid line) corresponding to the sum of the amount offirst exposure (dot line) and the amount of second exposure (dash line)can be made uniform.

[0061]FIG. 15 is a graph chart of simulation of an exposure intensitydistribution of an X-ray diffraction image to be absorbed by a workpiece7, under the same conditions as of FIG. 8 except that: the absorbingmember 12 of the mask is made of 0.45-micron thickness molybdenum; theline-and-space of the mask pattern is at a 0.1 (micron):0.1 (micron)ratio; and the exposure gap is 19 microns. FIG. 16 is a graph whereinthe exposure intensity distribution of FIG. 15 and an exposure intensitydistribution, resulting from shifting it by an amount corresponding to ahalf period of the mask pattern, are added together. As shown in FIG.16, a uniform exposure intensity distribution can be produced.

[0062] It is to be noted here that, by setting the amounts of the firstand second exposures as desired, various exposure amount distributionscan be provided as desired.

[0063] Specific examples of the present invention will be describedbelow.

FIRST EXAMPLE

[0064] A fine pattern of 0.05-micron line-and-space formed in the mannerdescribed with reference to FIGS. 13 and 14 as well as a rough patternwith a large opening as produced in accordance with a conventionalmethod so that at least a portion where the fine pattern should be leftwas opened, were photoprinted respectively with exposure amounts eachbeing at a level lower than the exposure threshold of a positive typeresist upon a workpiece but, when combined, the sum of them being at alevel higher than the exposure threshold. The rough pattern mask had anX-ray absorbing member of 0.5-micron thickness tungsten, having anopening of 0.375 micron width formed therein. The rough pattern wasexposed by use of an exposure apparatus such as shown in FIG. 1, with anexposure gap of 10 microns. The exposure amount ratio between the finepattern and the rough pattern was 0.5:1. The result is shown in FIG. 4,wherein a fine pattern of 0.05-micron line-and-space was left only at adesired position.

[0065] In addition to the dual exposure of the fine pattern and therough pattern described above, a second rough pattern having an openingof 0.2-micron width was photoprinted, with the center of the 0.2-micronopening being registered with the center position of the above-described0.375-micron width opening, and at an exposure amount ratio 1. Theresist was left in a clear pattern shape of 0.25 micron in width.

[0066]FIG. 17 is a a graph chart of simulation of an X-ray exposureamount distribution to be absorbed by a workpiece 7 in a case where: themask has an absorbing member 12 of 0.5-micron thickness tungsten; arough pattern having a mask pattern opening of 0.5 micron width is setat an exposure gap of 10 microns; and the exposure process is performedby use of an X-ray exposure apparatus of the conditions similar to thoseof FIG. 8 example. There are peaks of exposure intensity due to Fresneldiffraction, at opposite ends of the opening of the rough pattern. FIG.18 is a graph of a combined exposure amount distribution in an examplewhere the fine pattern of FIG. 16 and the rough pattern of FIG. 17 arephotoprinted at an exposure amount ratio of 0.5:1, while the patternsare aligned with each other so that the exposure amount peak positionsof them are registered with each other. If the exposure threshold is setat an exposure intensity 1.0, five fine patterns of 0.05-micronline-and-space can be left at the opening of the rough pattern, in agood state. Namely, an edge of a mask pattern produced on the basis of aconventional technique can be fixed at the precision and resolution of afine pattern. Therefore, the precision and resolution of a pattern to beformed can be improved.

SECOND EXAMPLE

[0067] A fine pattern of 0.05-micron line-and-space formed in the mannerdescribed with reference to FIG. 10 as well as a pattern (rough pattern)of 0.1 micron width as produced in accordance with a conventional methodwere photoprinted respectively with exposure amounts each being at alevel lower than the exposure threshold of a positive type resist upon aworkpiece but, when combined, the sum of them being at a level higherthan the exposure threshold. The rough pattern mask had an X-rayabsorbing member of 0.5-micron thickness tungsten, having an opening of0.1 micron width formed therein. The exposure process was performedwhile the fine pattern and the rough pattern were aligned with eachother so that the peak of Fresnel diffraction of the rough pattern andthe peak of Fresnel diffraction of the fine pattern were registered witheach other, and it was done at an exposure amount ratio of 0.5:1. Theexposure gap was 10 microns.

[0068] In this example, a mask having an opening of 0.05-micron widthproduced by use of a conventional technique as well as a rough patternhaving an opening of 0.1-micron width with which a pattern of0.05-micron width could not be resolved when used with a conventionalapparatus, due to a too low contrast, were used. Also, the exposureprocess was performed while keeping the Fresnel diffraction peak of therough pattern and the Fresnel diffraction peak of the fine patternregistered with each other. Thus, by use of a mask produced inaccordance with a conventional technique as well as by use of aconventional exposure apparatus, a high contrast can be attained and theresolving power can be improved thereby.

[0069]FIG. 19 is a a graph chart of simulation of an exposure intensitydistribution upon a workpiece in a case where the exposure process isperformed under the conditions like those of FIG. 17, except for thatthe absorbing member 12 of the mask has an opening of 0.1 micron width.FIG. 20 is a graph of a combined exposure amount distribution in anexample where the fine pattern of FIG. 11 and the rough pattern of FIG.19 are photoprinted at an exposure amount ratio of 0.5:1, while thepatterns are aligned with each other so that the exposure amount peakpositions of them are registered with each other. If the exposurethreshold is set at an exposure intensity 1.0, an isolated line of 0.05micron (fine pattern) can be left at the position corresponding to theopening of the rough pattern, in a good state. Namely, while using amask pattern produced in accordance with a conventional technique, apattern that can not be resolved by a conventional method, can beresolved.

THIRD EXAMPLE

[0070] A fine pattern of 0.05-micron line-and-space formed in the mannerdescribed with reference to FIGS. 13 and 14 as well as a rough patternas produced in accordance with a conventional method were photoprintedrespectively with exposure amounts each being at a level lower than theexposure threshold of a positive type resist upon a workpiece but, whencombined, the sum of them being at a level higher than the exposurethreshold. The rough pattern mask had an X-ray absorbing member of0.5-micron thickness tungsten, having an opening of 0.25 micron widthformed therein. The rough pattern and the fine pattern were exposed atan exposure amount ratio of 0.5:1. The exposure gap for the roughpattern exposure was 10 microns. As a result of the above, a patternhaving two lines of 0.05-micron width disposed at a spacing of 0.05micron, was resolved, whereas the same could not be resolved by use of aconventional mask and a conventional apparatus.

[0071]FIG. 21 is a a graph chart of simulation of an exposure intensitydistribution upon a workpiece in a case where the exposure process isperformed under the conditions like those of FIG. 17, except for thatthe absorbing member 12 of the mask has an opening of 0.25 micron width.FIG. 22 is a graph of a combined exposure amount distribution in anexample where the fine pattern of FIG. 16 and the rough pattern of FIG.21 are photoprinted at an exposure amount ratio of 0.5:1. If theexposure threshold is set at an exposure intensity 1.0, two finepatterns of 0.05 micron width can be left at the position correspondingto the opening of the rough pattern, in a good state. Namely, whileusing a mask pattern produced in accordance with a conventionaltechnique, a pattern that can not be resolved by a conventional method,can be resolved.

FOURTH EXAMPLE

[0072] A fine pattern of 0.05-micron line-and-space formed in the mannerdescribed with reference to FIG. 7 as well as a rough pattern asproduced in accordance with a conventional method were photoprintedrespectively with exposure amounts each being at a level lower than theexposure threshold of a positive type resist upon a workpiece but, whencombined, the sum of them being at a level higher than the exposurethreshold.

[0073] The rough pattern mask as designed should have an X-ray absorbingmember of 0.35-micron thickness tungsten, having three openings withline-and-space of 0.05 (micron)/0.05 (micron) ratio. However, the roughpattern mask actually produced had tungsten of 0.35 micron thicknessexactly as designed, whereas the line-and-space of the openings was 0.06(micron)/0.04 (micron) and was narrow.

[0074] A mask expected as designed can be resolved singly when Fresneldiffraction is used, like the cases of FIGS. 7 and 8. However, with useof the actually produced rough pattern singly, the pattern could not beresolved even the Fresnel diffraction was used. But, when the finepattern of 0.05-micron line-and-space formed in the manner describedwith reference to FIG. 7 and the rough pattern produced as describedabove, were exposed at an exposure amount ratio 0.35:1, a pattern asdesired was resolved. Namely, in accordance with this example, any faultof a mask such as a production error can be compensated.

[0075]FIG. 23 is a graph chart of simulation of the state of exposurewhere a mask expected as designed is used. Namely, it shows the resultof simulation of an exposure intensity distribution upon a workpiece ina case where the exposure process is performed under the conditions likethose of FIG. 17, except for that: the mask absorbing member 12 has athickness 0.035 micron and has three openings of line-and-space of 0.05(micron)/0.05 (micron); and the exposure gap is 4 microns. FIG. 24 is agraph of a combined exposure amount distribution in an example where thefine pattern of FIG. 8 and the rough pattern of FIG. 23 are exposed atan exposure amount ratio of 0.35:1. There is substantially no differencebetween the exposure distribution of FIG. 23 and that of FIG. 24.

[0076]FIG. 25 is a graph chart of simulation of the state of exposurewhere a mask such as actually produced as described above, is used.Namely, it shows the result of simulation of an exposure intensitydistribution upon a workpiece in a case where the exposure process isperformed under the conditions like those of FIG. 23, except for thatthe line-and-space of the mask absorbing member 12 is 0.06 (micron)/0.04(micron). It is very difficult to resolve three patterns.

[0077]FIG. 26 is a graph of a combined exposure amount distribution inan example where the fine pattern of FIG. 8 and the rough pattern ofFIG. 25 are exposed at an exposure amount ratio of 0.35:1. If theexposure threshold is set at an exposure intensity 0.7-0.8, three finepatterns of 0.05-micron width can be left at the position of the openingof the rough pattern, in a good state. Therefore, as described above,through the dual exposure of a fine pattern and a rough pattern, apattern which otherwise can not be resolved (or to do so is verydifficult) only by use of a rough pattern, can be resolved. It is to benoted that, if the rough pattern has no fault so that it can be resolvedsingly, there will be no difference between the single exposure and thedual exposure, as shown in FIG. 24. In accordance with this embodiment,as described above, regardless of the presence/absence of a fault ordeflect in the rough pattern, a fine pattern can be formed in a goodstate. Thus, any fault of the mask can be compensated for.

[0078] [Alternative Forms]

[0079] As regards the fine pattern, in the embodiments described above,a pattern with a line-and-space (period) which is a half of that of amask pattern is printed on the basis of the X-ray interference. However,a pattern with a line-and-space which is 1/n (where n is an integer notless than 3) of that of the mask pattern, may be photoprinted, throughthe X-ray interference.

[0080] The mask for a rough pattern and the mask for a fine pattern mayhave X-ray absorbing members of either the same material or differentmaterials, and they may have either the same thickness or differentthicknesses. In relation to the fine pattern mask, the material orthickness of the absorbing member should be selected so that an exposureintensity distribution of a period of unit magnification or 1/nmagnification (n is an integer) can be produced on the basis ofinterference of X-rays. However, in relation to the rough patterntransfer, on the other hand, there is no necessity of satisfying theinterference condition as in the case of the fine pattern. Therefore, anabsorbing member and a thickness that enables high contrast, can beselected. This prevents fogging in the pattern transfer.

[0081] The fine-pattern transfer and the rough-pattern transfer may beperformed either with the same exposure gap or with different exposuregaps. For the fine-pattern transfer, an exposure gap that satisfies acondition for interference should be selected. On the other hand, forthe rough-pattern transfer, since it is not necessary to satisfy thecondition for interference as in the case of the fine pattern, anexposure gap that attains high pattern correctness can be selected.Alternatively, an exposure gap with which a peak position of exposureintensity of a fine pattern and a peak position of exposure intensity ofa rough pattern can be registered with each other, may be selected.

[0082] The exposure amount for the rough pattern and the exposure amountfor the fine pattern can be determined as desired, in accordance withexperiments or calculations.

[0083] While the fine pattern comprises a stripe-like pattern in theembodiments described above, it may be a checkered pattern such as shownin FIG. 27 or a grid-like pattern such as shown in FIG. 28.

[0084]FIG. 29 is a schematic view corresponding to a section taken alonga line A-A′ or B-B′ in FIG. 27, where an X-ray proximity exposure isperformed by use of a mask having the checkered pattern of FIG. 27.Denoted in the drawing at 7 is a wafer, and denoted at 8 is a resistapplied to the wafer. Denoted at 11 is a mask membrane, and denoted at12 is an absorbing member. The waves upon the resist 8 depict an X-rayintensity distribution thereupon.

[0085]FIG. 30 is a schematic and plan view for explaining an X-rayintensity distribution upon the resist 8 of FIG. 29, to be produced bythe exposure process of FIG. 29.

[0086] In the embodiments described above, a mask is manufactured inaccordance with a conventional mask manufacturing technique, and anexposure process for the mask is performed by use of a conventionalexposure apparatus. It is to be noted here that the present inventioncan still be applied when the mask manufacturing technique is improvedor the precision and resolution of an exposure apparatus is improved,and the invention can be embodied by use of such improved maskmanufacturing technique or improved exposure apparatus. Namely, whileusing a mask manufacturing technique and an exposure apparatus which areavailable at that time, a precision and a resolution, higher than thoseattainable with an ordinary exposure method, can be accomplished withthe present invention.

[0087] Next, an embodiment of a device manufacturing method which isbased on an exposure method described above, will be explained.

[0088]FIG. 31 is a flow chart of procedure for manufacture ofmicrodevices such as semiconductor chips (e.g. ICs or LSIs), liquidcrystal panels, CCDs, thin film magnetic heads or micro-machines, forexample.

[0089] Step 1 is a design process for designing a circuit of asemiconductor device. Step 2 is a process for making a mask on the basisof the circuit pattern design. Step 3 is a process for preparing a waferby using a material such as silicon. Step 4 is a wafer process (called apre-process) wherein, by using the so prepared mask and wafer, circuitsare practically formed on the wafer through lithography. Step 5subsequent to this is an assembling step (called a post-process) whereinthe wafer having been processed by step 4 is formed into semiconductorchips. This step includes an assembling (dicing and bonding) process anda packaging (chip sealing) process. Step 6 is an inspection step whereinoperation check, durability check and so on for the semiconductordevices provided by step 5, are carried out. With these processes,semiconductor devices are completed and they are shipped (step 7).

[0090]FIG. 32 is a flow chart showing details of the wafer process.

[0091] Step 11 is an oxidation process for oxidizing the surface of awafer. Step 12 is a CVD process for forming an insulating film on thewafer surface. Step 13 is an electrode forming process for formingelectrodes upon the wafer by vapor deposition. Step 14 is an ionimplanting process for implanting ions to the wafer. Step 15 is a resistprocess for applying a resist (photosensitive material) to the wafer.Step 16 is an exposure process for printing, by exposure, the circuitpattern of the mask on the wafer through the exposure apparatusdescribed above. Step 17 is a developing process for developing theexposed wafer. Step 18 is an etching process for removing portions otherthan the developed resist image. Step 19 is a resist separation processfor separating the resist material remaining on the wafer after beingsubjected to the etching process. By repeating these processes, circuitpatterns are superposedly formed on the wafer.

[0092] With these processes, high density microdevices can bemanufactured.

[0093] In the embodiments of the present invention as described above,by superposition of different exposure patterns, any manufacturingerrors of mask patterns can be averaged. Also, among transferred imagesof different exposure patterns, a portion with a higher resolution isemphasized, by which the precision or resolution power can be improved.Further, under such conditions where the contrast is too low andresolution is difficult to accomplish when a conventional mask and aconventional exposure method are used, the present invention enables theresolution.

[0094] As described, the present invention enables a higher precisionand a higher resolution by use of an ordinary mask manufacturing methodand an ordinary exposure apparatus which are available at that time(currently or in future), than the precision or resolution attainablesimply with such mask manufacturing method or exposure apparatus.

[0095] A fine pattern of smallest linewidth less than the resolutionlimit, attainable with a combination of an ordinary exposure apparatusand a currently available mask, may be printed on the basis of theinterference of X-rays. This may be combined with exposure of a roughpattern as can be produced by use of a currently available maskmanufacturing technique. The fine pattern exposure and the rough patternexposure may be performed respectively at individual exposure amounts,each being at a level lower than the exposure amount threshold of aresist upon a workpiece but, when combined, the sum of them being at alevel higher than the exposure threshold. This enables resolving asmallest linewidth of the fine pattern. There are various advantageouseffects involved, such as follows.

[0096] (1) Since the final pattern position strongly depends on thetransfer position of the fine pattern, there occurs substantially nopositional error provided that the transfer position of the fine patternis correct.

[0097] (2) Since the final pattern image strongly depends on atransferred image of the fine pattern, a defect in the rough pattern, ifany, is not easily transferred provided that the transferred image ofthe fine pattern is uniform.

[0098] (3) Since the transferred image of the fine pattern is formedthrough interference between plural patterns, a particular defect in thefine pattern mask, if any, is not substantially transferred.

[0099] While the invention has been described with reference to thestructures disclosed herein, it is not confined to the details set forthand this application is intended to cover such modifications or changesas may come within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. An exposure method for exposing a workpiece in aproximity exposure system, said method comprising: a first exposure stepfor printing, by exposure, an image of a first mask pattern on apredetermined portion of the workpiece; and a second exposure step forprinting, by exposure, an image of a second mask pattern, different fromthe first mask pattern, on the predetermined portion of the workpiece;wherein exposures in said first and second exposure steps are performedsuperposedly, prior to a development process.
 2. A method according toclaim 1, wherein the first mask pattern is used to form, in thepredetermined portion of the workpiece, a fine pattern having apredetermined smallest linewidth, and wherein the second mask pattern isused to form, in the predetermined portion of the workpiece, a roughpattern having a smallest linewidth larger than that of the first maskpattern.
 3. A method according to claim 2, wherein each of the firstmask pattern and the fine pattern has a periodic structure.
 4. A methodaccording to claim 3, wherein the first mask pattern comprises one of aline-and-space pattern, a checkered pattern and a grid-like pattern. 5.A method according to claim 4, wherein the fine pattern and the firstmask pattern have the same period.
 6. A method according to claim 4,wherein the fine pattern has a period corresponding to 1/n of the periodof the first mask pattern, where n is an integer not less than
 2. 7. Amethod according to claim 6, wherein said first exposure step isperformed while placing the first mask pattern at a first position andat a second position shifted from the first position by an amountcorresponding to a half of the period of the first mask pattern.
 8. Amethod according to claim 4, wherein the fine pattern is based on aFresnel diffraction image of the first mask pattern.
 9. A methodaccording to claim 8, wherein the rough pattern also is based on aFresnel diffraction image, and wherein exposures in said first andsecond exposure steps are performed so that a peak position of anexposure intensity of the rough pattern and a peak position of anexposure intensity of the fine pattern are registered with each other.10. A method according to claim 2, wherein the second mask pattern has asmallest linewidth not less than twice of the smallest linewidth of thefine pattern.
 11. A method according to claim 2, wherein the second maskpattern has a smallest linewidth substantially the same as that of thefine pattern.
 12. A method according to claim 1, wherein the first andsecond exposure steps are performed with different spacings,respectively, held between the mask and the workpiece.
 13. A methodaccording to claim 1, wherein the first and second mask patterns haveabsorbing material patterns, respectively, which are different inthickness from each other.
 14. A method according to claim 1, wherein atleast one of said first and second exposure steps is performed by use ofX-rays.
 15. A mask to be used for printing a pattern on a workpiece byexposure, said mask comprising: a membrane; and an absorbing materialpattern formed on the membrane in an array like a diffraction grating,said absorbing material pattern being set so that, when said mask isdisposed close to the workpiece with a predetermined gap maintainedtherebetween for execution of a proximity exposure, said absorbingmaterial pattern functions to form on the workpiece a diffraction imageof a period which is the same as the period of the absorbing materialpattern or which corresponds to 1/n of the period of the absorbingmaterial pattern where n is an integer not less than
 2. 16. A devicemanufacturing method, comprising the steps of: exposing a workpiece inaccordance with a proximity exposure procedure which includes a firstexposure process for printing, by exposure, an image of a first maskpattern on a predetermined portion of the workpiece, and a secondexposure process for printing, by exposure, an image of a second maskpattern, different from the first mask pattern, on the predeterminedportion of the workpiece, wherein exposures in the first and secondexposure processes are performed superposedly, prior to a developmentprocess; and developing the workpiece having been exposed, whereby acircuit pattern can be formed on the developed workpiece.
 17. A methodaccording to claim 16, wherein at least one of the first and secondexposure processes is performed by use of X-rays.
 18. A device,comprising: a substrate; and a circuit pattern formed on said substratein accordance with a procedure which includes (i) exposing a workpiecein accordance with a proximity exposure procedure including (a) a firstexposure process for printing, by exposure, an image of a first maskpattern on a predetermined portion of the workpiece, and (b) a secondexposure process for printing, by exposure, an image of a second maskpattern, different from the first mask pattern, on the predeterminedportion of the workpiece, wherein exposures in the first and secondexposure processes are performed superposedly, prior to a developmentprocess, and (ii) developing the workpiece having been exposed, wherebya circuit pattern can be formed on the developed workpiece.